1. Field of the Invention
The present invention relates to a reception synchronization circuit, a reception synchronization method and a receiver, and particularly to a reception synchronization circuit, a reception synchronization method, a receiver and a digital communication system for receiving a synchronous word or a unique word (hereinafter referred to as xe2x80x9cUWxe2x80x9d) which is used to establish synchronization and transmitted in a predetermined digital a pattern for transmission of digital data in a satellite communication and a mobile communication, for example, ICO system (Inmarsat-P Communication system), Iridium system, Global Star system, CDMAOne system, GSM (Global System for Mobile Communication) and W-CDMA system.
2. Description of the Related Art
Japanese Laid-open Patent Application No. Hei-1-256232 (prior art 1) discloses a synchronous word (UW) detection system. The synchronous word detection system comprises a line quality monitoring circuit for monitoring the quality of a line at any time with a controller of a TDMA (Time Division Multiple Access) satellite communication system and outputting line quality information at a predetermined time interval; a permission error number setting circuit which receives the line quality information and search/narrow mode information representing a reception synchronization process in the TDMA satellite communication system to make the permission error number (threshold value xcex5) of a synchronous word pattern of the satellite circuit variable in accordance with the quality of the line at a predetermined time interval and output it; a threshold value correlation detection circuit which receives the variable permission error from the permissible error number setting circuit, compares reception input data with a predetermined synchronous word pattern to establishment a correlation therebetween, and then outputting a coincidence pulse on the basis of an indicated permission error number xcex5; a storage circuit containing hardware information to exchange the permission error number setting circuit and the threshold value correlation detection circuit with each other on a real-time basis with the search/narrow information and the line quality information as an input address; and a logic circuit for performing AND between the coincidence pulse output from the threshold value correlation detection circuit and an aperture signal occurring at a normal position of a reception reference burst and outputting a synchronization detection pulse.
With the synchronous word system thus constructed, the UW detection can be efficiently performed on a real-time basis, and particularly it can perform the detection of UW efficiently when applied to an earth station having a compact-size antenna (1 to 2 m level) or an earth station in which deterioration of C/N due to attenuation of a rainfall remarkably appears.
Japanese Laid-open Patent Application No. Hei-3-46429 (prior art 2) discloses a unique word detection circuit apparatus including a code error rate measuring circuit for measuring the code error rate of reception data input and outputting the code error rate thus measured; a correlation detection threshold value calculation circuit for calculating and outputting the correlation detection threshold value (xcex5) corresponding to the code error rate; a microprocessor for outputting the correlation detection threshold value (xcex5) with a bus signal; and a unique word detection circuit for judging detection or miss detection of the correlation detection threshold value (xcex5) output from the microprocessor and the reception unique word and then outputting a unique word detection signal only when the unique word is detected.
With the unique word detection circuit apparatus thus constructed, loss of a unique word due to temporary deterioration of a code error rate , loss of reception data which is appendant to the loss of the unique word, and out-of-synchronization of a line can be prevented, thereby enhancing synchronization precision and reliability.
Japanese Laid-open Patent Application No. Hei-3-70226 (prior art 3) discloses maximum value detection selection means for selecting the reception signal having the maximum envelope level from reception signals which are received by plural antennas; a limiter circuit for dividing the reception signal having the maximum enveloped level thus selected by the maximum envelope level to normalize the reception signal having the maximum envelope level and set the envelope level to a predetermined value; means for calculating a complex correlation coefficient between the normalized reception signal and the complex signal corresponding to a specific frame synchronization pattern (UW); and comparison means for comparing the complex correlation coefficient with any threshold value level, wherein a frame synchronization detection circuit judges the reception signal as a frame synchronization pattern (UW) when the complex correlation coefficient is larger than the threshold value level.
Accordingly, in a reception apparatus using a diversity reception system, the effect of multi-path fading can be removed, and the detection precision of the frame synchronization pattern can be enhanced, so that there can be achieved a digital ground mobile radio (wireless) communication system having high reception precision.
Further, Japanese Laid-open Patent Application No. Hei-5-167630 (prior art 4) discloses a unique word detector including: a first delay detector for receiving as an input signal a signal obtained by subjecting quasi-synchronization and demodulation to a reception signal in which a unique word is inserted, and detecting the delay of the input signal; a unique word generator for generating a complex conjugate value of the unique word; a second delay detector for detecting the delay of the output of the unique word generator; a mutual correlator for taking a mutual correlation between the first and second delay detectors; and a level detector for comparing the output of the mutual correlator with a predetermined threshold value to detect the maximum value thereof.
With the unique word detector thus constructed, the input signal is subjected to the delay detection by the first delay detector to remove a frequency offset therefrom, and then input to the mutual correlator while a frequency offset is removed therefrom. Therefore, even when an input signal has large frequency instability, it is unnecessary to arrange many mutual correlators in parallel for use unlike the prior art, and thus the circuit scale can be greatly reduced. In addition, only one input signal is supplied to the level detector, so that the error detection probability can be reduced.
However, in the prior arts 1 to 4, the detection means of UW itself is shown, and it is applicable to a case where a received status is surely stable. However, when the received status is unstable, an error detection state occurs in UW itself, and thus UW detection which takes no consideration into the received status is little accurately performed. Further, a predetermined time is needed to detect the correlation of UW, and when the correlation of UW is taken although the received status is bad, time is merely wasted.
In the prior art 4, the level detection is performed by the quasi-synchronized and modulated signal of the reception signal and the unique word generator for generating the complex conjugate of UW. In this prior art, the level detection is accurate, however, the relationship between the level detection and the detection of UW is not clarified.
That is, when a reception frequency at which a reception signal exists is not known at an initial stage, the reception electric field intensity of the reception signal, that is, the reception power is first detected in order to establish an early synchronization between the reception frequency and the clock timing. At this time, in the case where the detection level of the reception power is judged on the basis of only one threshold value, the reception power is frequently judged as a noise if the threshold value is low, and thus erroneous detection frequently occurs. On the other hand, if the threshold value is high, the detection of the reception signal is missed. When the reception electric field intensity, that is, the detection level of the reception power is low, the probability that the detection error is increased is high, and thus it has been still required to perform accurate UW detection with a simple construction.
Therefore, an object of the present invention is to reduce the noise error detection and the signal miss detection by preparing one type or two or more types of threshold values for a movement average detected and calculated on the basis of received power and using an improved judgment algorithm.
Further, another object of the present invention is to enable adaptive controls in various communication states by storing various past detection situations to feed them back to the setting of a threshold value.
Still further, another object of the present invention is to keep an early realization of synchronization establishment by achieving a retrial of a maximum UW measurement.
In order to attain the above objects, according to a first aspect of the present invention, a reception synchronization circuit for receiving a unique word transmitted in a predetermined digital pattern is characterized by comprising: a detection circuit for outputting a reception movement average of a received signal; a memory for storing at least one threshold value for the movement average; comparison means for comparing the reception movement average and the threshold value of the movement average; and an UW correlation judgment circuit for detecting the unique word (hereinafter referred to as xe2x80x9cUWxe2x80x9d) when the reception movement average is higher than the threshold value of the movement average, and taking a correlation between the detected unique word and a predetermined digital pattern.
Further, according to a second aspect of the present invention, a reception synchronization circuit for receiving a unique word transmitted in a predetermined digital pattern is characterized by comprising: a detection circuit for detecting the received power of a received signal and outputting a movement average; a UW correlation judgment circuit for detecting the unique word (hereinafter referred to as xe2x80x9cUWxe2x80x9d) and taking a correlation between the unique word thus detected and a predetermined digital pattern; and a memory for storing two or more threshold values for the movement average, wherein after the two or more threshold values and the movement average are compared with each other, the correlation is taken by the UW correlation judgment circuit when the movement average is larger than the minimum values of the two or more threshold values.
In the above reception synchronization circuit, when the movement average is larger than the maximum threshold value of the two or more threshold values, the correlation is taken by the UW correlation judgment circuit. Further, when the movement average is smaller than the minimum threshold value of the two or more threshold values, the minimum threshold value is increased.
According to a third aspect of the present invention, a receiver according to the present performs data detection based on the TDMA system by using the above reception synchronization circuit.
According to a fourth aspect of the present invention, a reception synchronization method for receiving a unique word (hereinafter referred to as xe2x80x9cUWxe2x80x9d) transmitted in a predetermined digital pattern is characterized by comprising the steps of: detecting the received power of a received signal; calculating the mean square of the received power and outputting a reception movement average; reading out from a memory the minimum threshold value and the maximum threshold value which are stored in the memory in advance; comparing the reception movement average with the minimum threshold value; detecting the UW when the reception movement average is larger than the minimum threshold value; and taking a correlation between the UW and the predetermined digital pattern.
In the above reception synchronization method, when the reception movement average is judged to be smaller than the minimum threshold value as a result of the comparison between the reception movement average and the minimum threshold value, the minimum threshold value is reduced to search a next reception channel. Further, in the above reception synchronization method, when the reception movement average is judged to be larger than the minimum threshold value and smaller than the maximum threshold value as a result of the comparison of the reception movement average, the minimum threshold value and the maximum threshold value, the maximum threshold value is reduced to perform a correlation calculation between the received signal and the UW.
According to a fifth aspect of the present invention, a digital communication system according to the present invention performs synchronization by using the above reception synchronization method.
Specifically, the signal detection and synchronization are established by the power detection and the UW (Unique Word) correlation. At this time, the judgment of the detection is performed on the basis of the threshold value, and two kinds of threshold values are prepared for the power detection to enhance the early search and the detection probability. Further, the past results are beforehand stored to suitably renew the threshold value.